For many years, liquid crystal displays (LCDs) were commonly backlit using cold cathode fluorescent lamps (CCFLs). However, with increasing frequency, CCFL backlights are being replaced with light emitting diode (LED) backlights.
Typically, the LEDs that are used to backlight an LCD are driven by one or more pulse-width modulated (PWM) drive signals. The brightness of the backlight can then be regulated by sensing the light production of the backlight and regulating the duty cycle of the PWM drive signals in response thereto.
Often, the light production of a backlight is determined by means of a circuit 600 including a photodiode 602 (FIG. 6). The output of the photodiode 602 is coupled to a transimpedance amplifier 604, and the output of the transimpedance amplifier 604 is coupled to a low-pass filter (RC). However, this arrangement is not particularly suited for sensing the light production of a pulsating light source. For example, the input to the transimpedance amplifier 602 can oscillate—particularly when the feedback resistance (Rf) of the transimpedance amplifier 604 is large and the pulsating intensity of incident light on the photodiode 602 is low. Further, the time constant of the low-pass filter can result in a slow response time. The circuit 600 can also be expensive, as the resistor (R) and capacitor (C) of the low-pass filter cannot be practically realized in silicon.